RabbitMQ tutorial - "Hello World!"

Introduction

RabbitMQ is a message broker. In essence, it accepts messages from producers, and delivers them to consumers. In-between, it can route, buffer, and persist the messages according to rules you give it.

RabbitMQ, and messaging in general, uses some jargon.

Producing means nothing more than sending. A program that sends messages is a producer. We’ll draw it like that, with “P”: digraph { bgcolor=transparent; truecolor=true; rankdir=LR; node [style=“filled”]; // P1 [label=“P”, fillcolor=“#00ffff”]; }

A queue is the name for a mailbox. It lives inside RabbitMQ. Although messages flow through RabbitMQ and your applications, they can be stored only inside a queue. A queue is not bound by any limits, it can store as many messages as you like - it’s essentially an infinite buffer. Many producers can send messages that go to one queue - many consumers can try to receive data from one queue. A queue will be drawn like this, with its name above it: digraph { bgcolor=transparent; truecolor=true; rankdir=LR; node [style=“filled”]; // subgraph cluster_Q1 { label=“queue_name”; color=transparent; Q1 [label=“{||||}”, fillcolor=“red”, shape=“record”]; }; }

Consuming has a similar meaning to receiving. A consumer is a program that mostly waits to receive messages. On our drawings it’s shown with “C”: digraph { bgcolor=transparent; truecolor=true; rankdir=LR; node [style=“filled”]; // C1 [label=“C”, fillcolor=“#33ccff”]; }

Note that the producer, consumer, and broker do not have to reside on the same machine; indeed in most applications they don’t.

“Hello World”

(using the Java Client)

In this part of the tutorial we’ll write two programs in Java; a producer that sends a single message, and a consumer that receives messages and prints them out. We’ll gloss over some of the detail in the Java API, concentrating on this very simple thing just to get started. It’s a “Hello World” of messaging.

In the diagram below, “P” is our producer and “C” is our consumer. The box in the middle is a queue - a message buffer that RabbitMQ keeps on behalf of the consumer.

The Java client library
RabbitMQ speaks AMQP, which is an open,
general-purpose protocol for messaging. You will need to install
the Java client package for Pivotal RabbitMQ to progress with
these tutorials.

Now we have the Java client and its dependencies, we can write some code.

Sending

We’ll call our message sender Send and our message receiver Recv. The sender will connect to RabbitMQ, send a single message, then exit.

The connection abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Here we connect to a broker on the local machine - hence the localhost. If we wanted to connect to a broker on a different machine we’d simply specify its name or IP address here.

Next we create a channel, which is where most of the API for getting things done resides.

To send, we must declare a queue for us to send to; then we can publish a message to the queue:

Sending doesn't work!
If this is your first time using RabbitMQ and you don't see the "Sent"
message then you may be left scratching your head wondering what could
be wrong. Maybe the broker was started without enough free disk space
(by default it needs at least 1Gb free) and is therefore refusing to
accept messages. Check the broker logfile to confirm and reduce the
limit if necessary. The configuration
file documentation will show you how to set disk_free_limit.

Receiving

That’s it for our sender. Our receiver is pushed messages from RabbitMQ, so unlike the sender which publishes a single message, we’ll keep it running to listen for messages and print them out.

Note that we declare the queue here, as well. Because we might start the receiver before the sender, we want to make sure the queue exists before we try to consume messages from it.

We’re about to tell the server to deliver us the messages from the queue. Since it will push us messages asynchronously, we provide a callback in the form of an object that will buffer the messages until we’re ready to use them. That is what QueueingConsumer does.